An engine air/fuel control system modulates the flow of fuel delivered to the engine with a modulation signal (100, 144) The feedback variable generated (210-228) from a two-state exhaust gas oxygen sensor (16) corrects the fuel flow (156). During each of a plurality of pre-determined intervals, the fuel flow is biased with a rich offset (342). Amplitude of the modulation signal is corrected by a difference between the feedback variable generated during two successive occurrences of the predetermined interval (346-378).